Remove letters-related code (#175)

This deletes a big ol' chunk of code related to letters. It's not everything—there are still a few things that might be tied to sms/email—but it's the the heart of letters function. SMS and email function should be untouched by this.

Areas affected:

- Things obviously about letters
- PDF tasks, used for precompiling letters
- Virus scanning, used for those PDFs
- FTP, used to send letters to the printer
- Postage stuff

app/dao/uploads_dao.py

@@ -129,23 +129,3 @@ def dao_get_uploads_by_service_id(service_id, limit_days=None, page=1, page_size
     ).order_by(
         desc("processing_started"), desc("created_at")
     ).paginate(page=page, per_page=page_size)
-
-
-def dao_get_uploaded_letters_by_print_date(service_id, letter_print_date, page=1, page_size=50):
-    return db.session.query(
-        Notification,
-    ).join(
-        Template, Notification.template_id == Template.id
-    ).filter(
-        Notification.service_id == service_id,
-        Notification.notification_type == LETTER_TYPE,
-        Notification.api_key_id.is_(None),
-        Notification.status != NOTIFICATION_CANCELLED,
-        Template.hidden.is_(True),
-        _get_printing_day(Notification.created_at) == letter_print_date.date(),
-    ).order_by(
-        desc(Notification.created_at)
-    ).paginate(
-        page=page,
-        per_page=page_size,
-    )